Recently, the interference between adjacent bits poses a problem when increasing the recording density of a magnetic recording medium of a hard disk drive. A bit patterned medium physically divided in the bit direction has been proposed as a high-density magnetic recording medium capable of suppressing a thermal decay phenomenon by which recording marks disappear at room temperature, and suppressing medium noise from adjacent marks.
As the process of manufacturing the bit patterned medium, the development of a nanoimprinting method that duplicates a large amount of patterns has been advanced in order to inexpensively mass-produce media. Therefore, the technique of manufacturing a master as the start point of the duplication is the key. However, the bit patterned medium is expected as a high-density magnetic recording medium exceeding 2 Tbpsi (2 Terabit pitch per square inches), and the magnetic material dot period of one bit is 20 nm or less. Accordingly, the medium requires ultrafine patterns that are hardly formed by the photolithography technique having been developed for use in the semiconductor field or for the development of an optical disk master. Recently, a bit patterned medium master has been developed by using electron-beam lithography. Unfortunately, electron-beam lithography has the serious problems, i.e., the throughput is low, and the resolution decreases due to the proximity effect or the like.
Self-organization lithography using a diblock copolymer is a method capable of inexpensively forming fine patterns of a few nm to a few ten nm by using a micro phase separation structure (e.g., a lamellar structure, cylinder structure, or sphere structure). An imprinting master can be manufactured by etching a substrate by using a fine pattern having this self-organizing structure as a mask. However, to manufacture a bit patterned medium master by using self-organizing lithography, patterning must be performed using a layout that enables recording and reproduction in a hard disk drive.
As a method of forming a self-organizing structure in a necessary region of the medium surface, a method of preforming a desired physical guide groove and forming a dot as a micro phase separation structure in the groove has been proposed. When forming a concentric groove structure on a medium, forming a sphere-phase, self-organizing structure in this guide groove, and transferring a pattern onto a substrate, the sphere portion is transferred as a dot corresponding to one bit, and this makes it possible to manufacture a master for a data area of the bit patterned medium. In the guide groove, the dot array takes a hexagonal close-packed structure. On the other hand, to allow this bit patterned medium to function as a magnetic recording medium, it is necessary to form a pattern that forms not only a recording area but also a servo signal area in which information of the relative position of a recording/reproduction head and the track central position, track data information, and sector data information are embedded. The servo area includes a preamble portion for generating a sync signal, an address portion containing sector information and cylinder information, and a burst portion for obtaining a positioning signal. This servo area requires not a simple linear groove but a shape area corresponding to each signal characteristic, and hence is hardly formed by a regular self-organizing pattern. In addition, the servo area does not simply extend in the radial direction from the disk center, but is formed along the locus of a swing arm of a head.
The preamble portion is an essential area for obtaining a sync signal for signal recording and reproduction. If the signal quality of this area is low, it is impossible to input a reproduction signal to a PLL (Phase Locked Loop) and generate a reproduction clock signal. The address portion is an essential area for obtaining, e.g., the cylinder number of the data area. If the signal quality of this area is low, it is impossible to find (seek) a desired data area during recording/reproduction. In the present hard disk magnetic recording medium, a magnetic layer is formed on a flat disk substrate such as glass, and a continuous magnetic material mark is formed as a servo signal mark from the inner circumference to the outer circumference by using a servo writer apparatus or the like. The width of this servo signal mark in the disk circumferential direction continuously increases from the inner circumference toward the outer circumference, because the hard disk drive uses the CAV (Constant Angular Velocity) method in which the rotational angular velocity is constant during mark recording/reproduction. The bit patterned medium can be manufactured by forming the above-described servo signal pattern as a three-dimensional shape on an imprinting master, and transferring the pattern onto the medium. When manufacturing the bit patterned medium by self-organizing lithography, therefore, the servo portion must be formed by using a self-organizing pattern in the same manner as for the data portion. Accordingly, the servo signal area pattern must be formed by preforming a guide groove in a prospective servo signal pattern area of the present continuous magnetic recording film medium, and forming a self-organizing pattern in the groove. The preamble portion and address portion can be formed by forming a guide groove in an area extending from the inner circumference to the outer circumference, and transferring a pattern formed in the guide groove onto the medium by the nanoimprinting method. Since, however, the groove width changes from the inner circumferential side to the outer circumferential side, an area in which the number of self-organizing dot rows discontinuously changes forms, and this makes it very difficult to uniformly fill the dots. A magnetic medium manufactured using this master has the problem that there is a portion where a large defect occurs depending on the radial position, and the signal quality degrades and makes recording/reproduction difficult in the preamble area. Also, the magnetic material area (the filling ratio) is smaller (lower) than that of the continuous film medium. If there is a defective portion, therefore, the signal amplitude largely decreases and makes recording/reproduction difficult.